Publications

@article{Ruppert2022,

title = {Experimental Analysis of Tip Vibrations at Higher Eigenmodes of QPlus Sensors for Atomic Force Microscopy},

author = {M. G. Ruppert and D. Martin-Jimenez and Y. K. Yong and A. Ihle and A. Schirmeisen and A. J. Fleming and D. Ebeling},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2022/03/J22c.pdf},

doi = {10.1088/1361-6528/ac4759},

issn = {1361-6528},

year  = {2022},

date = {2022-02-10},

urldate = {2022-02-10},

journal = {Nanotechnology},

volume = {33},

issue = {18},

pages = {185503},

abstract = {QPlus sensors are non-contact atomic force microscope probes constructed from a quartz tuning fork and a tungsten wire with an electrochemically etched tip. These probes are self-sensing and offer an atomic-scale spatial resolution. Therefore, qPlus sensors are routinely used to visualize the chemical structure of adsorbed organic molecules via the so-called bond imaging technique. This is achieved by functionalizing the AFM tip with a single CO molecule and exciting the sensor at the first vertical cantilever resonance mode. Recent work using higher-order resonance modes has also resolved the chemical structure of single organic molecules. However, in these experiments, the image contrast can differ significantly from the conventional bond imaging contrast, which was suspected to be caused by unknown vibrations of the tip. This work investigates the source of these artefacts by using a combination of mechanical simulation and laser vibrometry to characterize a range of sensors with different tip wire geometries. The results show that increased tip mass and length cause increased torsional rotation of the tuning fork beam due to the off-center mounting of the tip wire, and increased flexural vibration of the tip. These undesirable motions cause lateral deflection of the probe tip as it approaches the sample, which is rationalized to be the cause of the different image contrast. The results also provide a guide for future probe development to reduce these issues.},

keywords = {Actuator, AFM, Cantilever, Multifrequency AFM, piezoelectric},

pubstate = {published},

tppubtype = {article}

}

@article{Moore2020,

title = {AFM Cantilever Design for Multimode Q Control: Arbitrary Placement of Higher-Order Modes},

author = {S. I. Moore and M. G. Ruppert and Y. K. Yong},

url = {https://ieeexplore.ieee.org/document/9006926},

doi = {10.1109/TMECH.2020.2975627},

year  = {2020},

date = {2020-02-21},

journal = { IEEE/ASME Transactions on Mechatronics},

pages = {1-6},

abstract = {In the fast growing field of multifrequency atomic force microscopy (AFM), the benefits of using higher-order modes has been extensively reported on. However, higher modes of AFM cantilevers are difficult to instrument and Q control is challenging owing to their high frequency nature. At these high frequencies, the latencies in the computations and analog conversions of digital signal processing platforms become significant and limit the effective bandwidth of digital feedback controller implementations. To address this issue, this article presents a novel cantilever design for which the first five modes are placed within a 200 kHz bandwidth. The proposed cantilever is designed using a structural optimization routine. The close spacing and low mechanical bandwidth of the resulting cantilever allows for the implementation of Q controllers for all five modes using a standard FPGA development board for bimodal AFM and imaging on higher-order modes.},

note = {Early Access},

keywords = {AFM, Cantilever, MEMS, Multifrequency AFM, Smart Structures, SPM, Vibration Control},

pubstate = {published},

tppubtype = {article}

}

@article{Harcombe2020,

title = {A review of demodulation techniques for multifrequency atomic force microscopy},

author = {D. M. Harcombe and M. G. Ruppert and A. J. Fleming},

editor = {T. Glatzel},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2020/02/J20b-reducedSize.pdf},

doi = {doi:10.3762/bjnano.11.8},

issn = {21904286},

year  = {2020},

date = {2020-01-07},

journal = {Beilstein Journal of Nanotechnology},

volume = {11},

pages = {76-97},

abstract = {This article compares the performance of traditional and recently proposed demodulators for multifrequency atomic force microscopy. The compared methods include the lock-in amplifier, coherent demodulator, Kalman filter, Lyapunov filter, and direct-design demodulator. Each method is implemented on a field-programmable gate array (FPGA) with a sampling rate of 1.5 MHz. The metrics for comparison include the sensitivity to other frequency components and the magnitude of demodulation artifacts for a range of demodulator bandwidths. Performance differences are demonstrated through higher harmonic atomic force microscopy imaging.},

keywords = {AFM, Demodulation, Multifrequency AFM, SPM},

pubstate = {published},

tppubtype = {article}

}

@article{Moore2019,

title = {Design and Analysis of Low-Distortion Demodulators for Modulated Sensors},

author = { S. I. Moore and M. G. Ruppert and D. M. Harcombe and A. J. Fleming and Y. K. Yong },

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2020/05/J19d-reduced.pdf},

doi = {10.1109/TMECH.2019.2928592},

issn = {10834435},

year  = {2019},

date = {2019-07-17},

journal = {IEEE/ASME Transactions on Mechatronics},

volume = {24},

number = {4},

pages = {1861-1870},

abstract = {System-based demodulators in the form of a Kalman and Lyapunov filter have been demonstrated to significantly outperform traditional demodulators, such as the lock-in amplifier, in bandwidth sensitive applications, for example high-speed atomic force microscopy. Building on their closed loop architecture, this article describes a broader class of high-speed closed-loop demodulators. The generic structure provides greater flexibility to independently control the bandwidth and sensitivity to out-of-band frequencies. A linear time-invariant description is derived which allows the utilization of linear control theory to design the demodulator. Experimental results on a nanopositioner with capacitive sensors demonstrate the realization of arbitrary demodulator dynamics while achieving excellent noise rejection.},

keywords = {Demodulation, Multifrequency AFM, Nanopositioning, SPM},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2018b,

title = {Multimodal atomic force microscopy with optimized higher eigenmode sensitivity using on-chip piezoelectric actuation and sensing},

author = {M. G. Ruppert and S. I. Moore and M. Zawierta and A. J. Fleming and G. Putrino and Y. K. Yong},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2019/08/Ruppert_2019_Nanotechnology_30_085503.pdf},

doi = {https://doi.org/10.1088/1361-6528/aae40b},

year  = {2019},

date = {2019-01-02},

journal = {Nanotechnology},

volume = {30},

number = {8},

pages = {085503},

abstract = {Atomic force microscope (AFM) cantilevers with integrated actuation and sensing provide several distinct advantages over conventional cantilever instrumentation. These include clean frequency responses, the possibility of down-scaling and parallelization to cantilever arrays as well as the absence of optical interference. While cantilever microfabrication technology has continuously advanced over the years, the overall design has remained largely unchanged; a passive rectangular shaped cantilever design has been adopted as the industry wide standard. In this article, we demonstrate multimode AFM imaging on higher eigenmodes as well as bimodal AFM imaging with cantilevers using fully integrated piezoelectric actuation and sensing. The cantilever design maximizes the higher eigenmode deflection sensitivity by optimizing the transducer layout according to the strain mode shape. Without the need for feedthrough cancellation, the read-out method achieves close to zero actuator/sensor feedthrough and the sensitivity is sufficient to resolve the cantilever Brownian motion.},

keywords = {AFM, Cantilever, MEMS, Multifrequency AFM, Piezoelectric Transducers and Drives, Sensors, Smart Structures, SPM},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2018b,

title = {Self-sensing, estimation and control in multifrequency Atomic Force Microscopy. },

author = {M. G. Ruppert},

url = {https://royalsoc.org.au/images/pdf/journal/151-1-Ruppert.pdf},

issn = {0035-9173/18/010111-01},

year  = {2018},

date = {2018-06-01},

journal = {Journal & Proceedings of the Royal Society of New South Wales},

volume = {151},

number = {1},

pages = {111},

abstract = {Despite the undeniable success of the atomic force  microscope  (AFM),  dynamic  techniques still face limitations in terms of spatial resolution, imaging speed and high cost of acquisition. In order to expand the capabilities  of  the  instrument,  it  was  realized  that the information about the nano-mechanical properties  of  a  sample  are  encoded  over  a range of frequencies and the excitation and detection of higher-order eigenmodes of the micro-cantilever  open  up  further  informa-

tion  channels.  The  ability  to  control  these modes and their fast responses to excitation is believed to be the key to unravelling the true potential of these ethods. This work addresses  three  major  drawbacks  of  the standard AFM setup, which limit the feasibility of multi-frequency approaches.



First,   microelectromechanical   system (MEMS)  probes  with  integrated  piezoelectric layers is motivated, enabling the development of novel multimode self-sensing and self-actuating techniques. Specifically, these piezoelectric  transduction  schemes  permit the  miniaturization  of  the  entire  AFM towards  a  cost-effective  single-chip  device with nanoscale precision in a much smaller form factor than that of conventional macroscale instruments.



Second, the integrated actuation enables the development of multimode controllers which  exhibits  remarkable  performance  in arbitrarily  modifying  the  quality  factor  of multiple eigenmodes and comes with inherent  stability  robustness.  The  experimental results  demonstrate  improved  imaging  stability,  higher  scan  speeds  and  adjustable contrast  when  mapping  nano-mechanical properties of soft samples. 



Last, in light of the demand for constantly increasing  imaging  speeds  while  providing multi-frequency flexibility, the estimation of multiple components of the high-frequency deflection signal is performed with a linear time-varying multi-frequency Kalman filter. The chosen representation allows for an efficient high-bandwidth implementation on a Field  Programmable  Gate  Array. Tracking bandwidth, noise performance and trimodal AFM imaging on a two-component polymer sample  are  verified  and  shown  to  be  superior to that of the commonly used lock-in amplifier.},

keywords = {Cantilever, MEMS, Multifrequency AFM, Sensors, Smart Structures, SPM, System Identification, Vibration Control},

pubstate = {published},

tppubtype = {article}

}

@article{J18c,

title = {Lyapunov Estimation for High-Speed Demodulation in Multifrequency Atomic Force Microscopy},

author = {D. M. Harcombe and M. G. Ruppert and M. R. P. Ragazzon and A. J. Fleming},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2018/02/J18c.pdf},

doi = {10.3762/bjnano.9.47},

issn = {21904286},

year  = {2018},

date = {2018-02-28},

journal = {Beilstein Journal of Nanotechnology},

volume = {9},

pages = {490-498},

abstract = {An important issue in the emerging field of multifrequency atomic force microscopy (MF-AFM) is the accurate and fast demodulation of the cantilever-tip deflection signal. As this signal consists of multiple frequency components and noise processes, a lock-in amplifier is typically employed for its narrowband response. However, this demodulator suffers inherent bandwidth limitations as high frequency mixing products must be filtered out and several must be operated in parallel. Many MF-AFM methods require amplitude and phase demodulation at multiple frequencies of interest, enabling both z-axis feedback and phase contrast imaging to be achieved. This article proposes a model-based multifrequency Lyapunov filter implemented on a Field Programmable Gate Array (FPGA) for high-speed MF-AFM demodulation. System descriptions and simulations are verified by experimental results demonstrating high tracking bandwidths, strong off-mode rejection and minor sensitivity to cross-coupling effects. Additionally, a five-frequency system operating at 3.5MHz is implemented for higher harmonic amplitude and phase imaging up to 1MHz.},

keywords = {AFM, Multifrequency AFM},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2017b,

title = {Note: Guaranteed collocated multimode control of an atomic force microscope cantilever using on-chip piezoelectric actuation and sensing},

author = {M. G. Ruppert and Y. K. Yong},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2017/08/2017_JNote_RSI_Vol88_086109-2.pdf},

doi = {10.1063/1.4990451},

year  = {2017},

date = {2017-08-15},

journal = {Review of Scientific Instruments},

volume = {88},

number = {086109},

abstract = {The quality (Q) factor is an important parameter of the resonance of the microcantilever as it determines

both imaging bandwidth and force sensitivity. The ability to control the Q factor of multiple

modes is believed to be of great benefit for atomic force microscopy techniques involving multiple

eigenmodes. In this paper, we propose a novel cantilever design employing multiple piezoelectric

transducers which are used for separated actuation and sensing, leading to guaranteed collocation

of the first eight eigenmodes up to 3 MHz. The design minimizes the feedthrough usually observed

with these systems by incorporating a guard trace on the cantilever chip. As a result, a multimode

Q controller is demonstrated to be able to modify the quality factor of the first two eigenmodes over

up to four orders of magnitude without sacrificing robust stability.},

keywords = {AFM, Cantilever, MEMS, Multifrequency AFM, Piezoelectric Transducers and Drives, System Identification, Vibration Control},

pubstate = {published},

tppubtype = {article}

}

@article{Moore2017,

title = {Design and Characterization of Cantilevers for Multi-Frequency Atomic Force Microscopy},

author = {S. I. Moore and Y. K. Yong},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2017/01/MNL.2016.0586.pdf},

doi = {10.1049/mnl.2016.0586},

year  = {2017},

date = {2017-03-01},

journal = {Micro & Nano Letters},

volume = {12},

number = {5},

pages = {315-320},

abstract = {The experimental characterisation of a set of microcantilevers targeted at use in multi-frequency atomic force microscope is presented. The aim of this work is to design a cantilever that naturally amplifies its harmonic oscillations which are introduced by nonlinear probe–sample

interaction forces. This is performed by placing the modal frequencies of the cantilever at integer multiples of the first modal frequency. The developed routine demonstrates the placement of the frequency of the second to fifth mode. The characterisation shows a trend that

lower-order modes are more accurately placed than higher-order modes. With two fabricated designs, the error in the second mode is at most 2.26% while the greatest error in the fifth mode is at 10.5%.},

keywords = {Cantilever, MEMS, Multifrequency AFM, SPM},

pubstate = {published},

tppubtype = {article}

}

@article{Moore2017b,

title = {Multimodal cantilevers with novel piezoelectric layer topology for sensitivity enhancement},

author = {S. I. Moore and M. G. Ruppert and Y. K. Yong},

url = {https://www.precisionmechatronicslab.com/wp-content/uploads/2017/02/2190-4286-8-38.pdf},

year  = {2017},

date = {2017-02-06},

journal = {Beilstein Journal of Nanotechnology},

volume = {8},

pages = {358--371},

abstract = {Self-sensing techniques for atomic force microscope (AFM) cantilevers have several advantageous characteristics compared to the optical beam deflection method. The possibility of down scaling, parallelization of cantilever arrays and the absence of optical interference associated imaging artifacts have led to an increased research interest in these methods. However, for multifrequency AFM, the optimization of the transducer layout on the cantilever for higher order modes has not been addressed. To fully utilize an integrated piezoelectric transducer, this work alters the layout of the piezoelectric layer to maximize both the deflection of the cantilever and measured piezoelectric charge response for a given mode with respect to the spatial distribution of the strain. On a prototype cantilever design, significant increases in actuator and sensor sensitivities were achieved for the first four modes without any substantial increase in sensor noise. The transduction mechanism is specifically targeted at multifrequency AFM and has the potential to provide higher resolution imaging on higher order modes.},

keywords = {Cantilever, Multifrequency AFM, Piezoelectric Transducers and Drives, SPM},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2016,

title = {High-Bandwidth Demodulation in MF-AFM: A Kalman Filtering Approach},

author = {M. G. Ruppert and D. M. Harcombe and S. O. R. Moheimani},

doi = {10.1109/TMECH.2016.2574640},

year  = {2016},

date = {2016-12-01},

journal = {IEEE/ASME Transactions on Mechatronics},

volume = {21},

number = {6},

pages = {2705-2715},

abstract = {Emerging multifrequency atomic force microscopy (MF-AFM) methods rely on coherent demodulation of the cantilever deflection signal at multiple frequencies. These measurements are needed in order to close the z-axis feedback loop and to acquire complementary information on the tip-sample interaction. While the common method is to use a lock-in amplifier capable of recovering low-level signals from noisy backgrounds, its performance is ultimately bounded by the bandwidth of the low-pass filters. In light of the demand for constantly increasing imaging speeds while providing multifrequency flexibility, we propose to estimate the in-phase and quadrature components with a linear time-varying Kalman filter. The chosen representation allows for an efficient high-bandwidth implementation on a Field Programmable Gate Array (FPGA). Tracking bandwidth and noise performance are verified experimentally and trimodal AFM results on a two-component polymer sample highlight the applicability of the proposed method for MF-AFM.},

keywords = {Multifrequency AFM, SPM},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2016b,

title = {Multimode Q Control in Tapping-Mode AFM: Enabling Imaging on Higher Flexural Eigenmodes},

author = {M. G. Ruppert and S. O. R. Moheimani},

doi = {10.1109/TCST.2015.2478077},

year  = {2016},

date = {2016-07-01},

journal = {IEEE Transactions on Control Systems Technology},

volume = {24},

number = {4},

pages = {1149-1159},

abstract = {Numerous dynamic Atomic Force Microscopy (AFM) methods have appeared in recent years, which make use of the excitation and detection of higher order eigenmodes of the microcantilever. The ability to control these modes and their responses to excitation is believed to be the key to unraveling the true potential of these methods. In this work, we highlight a multi-mode Q control method that exhibits remarkable damping performance and stability robustness. The experimental results obtained in ambient conditions demonstrate improved imaging stability by damping non-driven resonant modes when scanning is performed at a higher eigenmode of the cantilever. Higher scan speeds are shown to result from a decrease in transient response time. },

keywords = {MEMS, Multifrequency AFM, SPM, System Identification, Vibration Control},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2016b,

title = {High-bandwidth Multimode Self-sensing in Bimodal Atomic Force Microscopy},

author = {M. G. Ruppert and S. O. R. Moheimani},

doi = {10.3762/bjnano.7.26},

year  = {2016},

date = {2016-02-24},

journal = {Beilstein Journal of Nanotechnology},

volume = {7},

pages = {284-295},

abstract = {Using standard microelectromechanical system (MEMS) processes to coat a microcantilever with a piezoelectric layer results in a versatile transducer with inherent self-sensing capabilities. For applications in multifrequency atomic force microscopy (MF-AFM), we illustrate that a single piezoelectric layer can be simultaneously used for multimode excitation and detection of the cantilever deflection. This is achieved by a charge sensor with a bandwidth of 10 MHz and dual feedthrough cancellation to recover the resonant modes that are heavily buried in feedthrough originating from the piezoelectric capacitance. The setup enables the omission of the commonly used piezoelectric stack actuator and optical beam deflection sensor, alleviating limitations due to distorted frequency responses and instrumentation cost, respectively. The proposed method benefits from a more than two orders of magnitude increase in deflection to strain sensitivity on the fifth eigenmode leading to a remarkable signal-to-noise ratio. Experimental results using bimodal AFM imaging on a two component polymer sample validate that the self-sensing scheme can therefore be used to provide both the feedback signal, for topography imaging on the fundamental mode, and phase imaging on the higher eigenmode.},

keywords = {MEMS, Multifrequency AFM, Piezoelectric Transducers and Drives, Vibration Control},

pubstate = {published},

tppubtype = {article}

}

@article{Ruppert2016b,

title = {A Kalman Filter for Amplitude Estimation in High-Speed Dynamic Mode Atomic Force Microscopy},

author = {M. G. Ruppert and K. S. Karvinen and S. L. Wiggins and S. O. R. Moheimani},

doi = {10.1109/TCST.2015.2435654},

year  = {2016},

date = {2016-01-01},

journal = {IEEE Transactions on Control Systems Technology},

volume = {24},

number = {1},

pages = {276-284},

abstract = {A fundamental challenge in dynamic mode atomic force microscopy (AFM) is the estimation of the cantilever oscillation amplitude from the deflection signal which might be distorted by noise and/or high-frequency components. When the cantilever is excited at resonance, its deflection is typically obtained via narrowband demodulation using a lock-in amplifier. However, the bandwidth of this measurement technique is ultimately bounded by the low-pass filter which must be employed after demodulation to attenuate the component at twice the carrier frequency. Furthermore, to measure the amplitude of multiple frequency components such as higher eigenmodes and/or higher harmonics in multifrequency AFM, multiple lock-in amplifiers must be employed. In this work, the authors propose the estimation of amplitude and phase using a linear time-varying Kalman filter which is easily extended to multiple frequencies. Experimental results are obtained using square-modulated sine waves and closed-loop AFM scans, verifying the performance of the proposed Kalman filter.},

keywords = {Multifrequency AFM, SPM, Vibration Control},

pubstate = {published},

tppubtype = {article}

}